What is Expense Ratio?

Legal Definition
The expense ratio of a stock or asset fund is the total percentage of fund assets used for administrative, management, advertising (12b-1), and all other expenses. An expense ratio of 1% per annum means that each year 1% of the fund's total assets will be used to cover expenses. The expense ratio does not include sales loads or brokerage commissions.

Expense ratios are important to consider when choosing a fund, as they can significantly affect returns. Factors influencing the expense ratio include the size of the fund (small funds often have higher ratios as they spread expenses among a smaller number of investors), sales charges, and the management style of the fund. A typical annual expense ratio for a U.S. domestic stock fund is about 1%, although some passively managed funds (such as index funds) have significantly lower ratios.

One notable component of the expense ratio of U.S. funds is the "12b-1 fee", which represents expenses used for advertising and promotion of the fund. 12b-1 fees are generally limited to a maximum of 1.00% per year (.75% distribution and .25% shareholder servicing) under Financial Industry Regulatory Authority Rules.
-- Wikipedia
Legal Definition
A total of fees paid periodically to a mutual fund divided by the total investment that shareholders or unit holders has. These fees are management fees, administrative fees, and operating expenses of a mutual fund. This amount must be reported in its annual report. It is deducted from the fund's income. It typically totals from one percent to two percent of the income. This number appears as a ratio.